Connecting Current Signals to a DAQ Device

Included in the Section

Before You Begin

This document provides step-by-step instructions for wiring and configuring your NI DAQ device for current measurements. Before you begin using your DAQ hardware, you must install your application development environment and NI-DAQmx driver software. Refer to the Installing NI LabVIEW and NI-DAQmx document for more information.

Analog Measurement Fundamentals

NI DAQ devices measure current by means of an internal, precision shunt resistor. Most devices can measure ± 20 mA, which maps to the output range of most current transducers and 4 mA to 20 mA current loops. Before making signal connections, check the specifications of your device. When making signal connections, you must be careful with the voltage level of the signal going into the device. You can make two types of connections.

Differential Measurements

Devices capable of different current measurements have two terminals per measurement channel: AI+ and AI-. Channels on most devices are isolated from earth ground and each other. You can connect ground-referenced or floating current sources to your device. Connect the positive side of the current source to the AI+ terminal, and connect the negative side of the current source to the AI– terminal. If you make a ground-referenced connection between the current source and the DAQ device, make sure the voltage on the AI+ and AI– connections are in the channel-to-earth safety voltage range to ensure proper operation of your device.

Figure 1. Connecting a Differential Current Measurement

If your device does not have channel-to-channel isolation, verify that the voltage levels on the AI+ and AI- side do not exceed the common-mode input range. Common-mode input range is the voltage input range with respect to the device’s ground reference (AI+ versus AI GND, AI- versus AI GND, or AI+ versus AI-).

Single-Ended Measurements

Devices capable of single-ended measurements have one terminal per measurement channel (AI) and a common ground terminal (COM). Connect the positive lead of the current signal to the AI terminal. Connect the negative lead to the COM terminal. Refer to Figure 2 for an illustration of how to connect single-ended current signals to your device.

Figure 2. Connecting a Single-Ended Current Measurement

Field Wiring Considerations

Environmental noise can seriously affect the measurement accuracy of the device if you do not take proper care when running signal wires between signal sources and the device.

Minimize noise pickup and maximize measurement accuracy by using individually shielded, twisted-pair wires to connect AI signals to the device. With this type of wire, the signals attached to the positive and negative input channels are twisted together and covered with a shield. You then connect this shield only at one point to the signal source ground. This kind of connection is required for signals traveling through areas with large magnetic fields or high electromagnetic interference.

AI X(+/-): Analog Input — This is usually followed by a number corresponding to a channel number. For instance, if your DAQ card has 16 analog input channels, your pinout diagram probably shows positive and negative terminals for AI 0 through AI 15 throughout the connector. The + and – symbols refer to differential input channel pairs.

AI GND: Analog Ground — This terminal is electrically connected to the ground of the instrumentation amplifier on your DAQ device.

COM: Common — This terminal is a common voltage reference for all channels on your DAQ device.

Configuring a Current Measurement

You can use MAX to quickly verify the accuracy of your measurement system Set Up. Using an NI-DAQmx Global Virtual Channel you can configure a current measurement without any programming. A virtual channel is a concept of the NI-DAQmx driver architecture used to represent a collection of device property settings that can include a name, a physical channel, input terminal connections, the type of measurement or generation, and scaling information.

Follow these steps to begin:

With MAX open, select Data Neighborhood and click Create New.

Select NI-DAQmx Global Virtual Channel and click Next.

Select Acquire Signals » Analog Input » Current

Figure 4. Creating an NI-DAQmx Virtual Channel

Select the physical channel that you will connect your current measurement to. A physical channel is a terminal or pin at which you can measure or generate an analog or digital signal. A single physical channel can include more than one terminal or pin, as in the case of a differential input channel.

Figure 5. Device Physical Channels

Click Next and enter a name for the global virtual channel or leave the default.

Click Finish and to see the following screen in MAX:

Figure 6. Setting Up a Current Channel in MAX

On the settings tab, type in the minimum and maximum current values you expect to read from your signal (-20 mA to 20 mA by default). By doing this, you can achieve more resolution in your measurement because the digital codes of the analog-to-digital converter are spread evenly across the analog input range. So, if you expect and input a smaller range, the current input difference between two consecutive codes is smaller.

Wiring a Current Signal to Your Device

The next step is to physically connect the current signal to your DAQ device.

Click the Connection Diagram tab in MAX to continue.

Figure 7. Current Connection Diagram

The connection diagram above indicates which pins on your DAQ device should be wired according to the physical channel you selected. In this example, a singled-ended input connection uses pins 0 and 9, corresponding to AI0 and COM on an NI 9203 C Series module.

Testing the Signal

With MAX still open, click back on the NI-DAQmx Global Channel tab and click on the Run button. The current value of your measurement is now displayed at the top of the screen.

Figure 8. Previewing a Current Measurement in MAX

You can choose to view the signal in tabular form or as a graph by selecting Graph from the Display Type dropdown. You also have the option of saving your NI-DAQmx Global Virtual Channel should you wish to refer to this configuration screen again in the future.